Use a very large scale, smooth random texture (e.g., Perlin with large feature size, low octaves, low persistence) as a mask. Tune it so you get nice linear stretches with a width about one mountain's worth and a length appropriate for a mountain range, rather than roundish blobs. Experiment with combining multiple Perlins with the same settings, but offset along the natural axis for mountain ranges, or with non-uniform XY scaling. Multiply this mask times your mountain range texture.